The concept of extraordinary rendition does not have a clear meaning in international law. It is not referred to in any treaty or international instrument of which I am aware. It has come to be understood as referring to the practice of forcibly transporting a person, usually alleged to be involved in terrorism, from one country to another without relying on the normal legal processes for the purposes of subjecting them to interrogation and other forms of treatment that include torture or cruel and degrading treatment.

Both elements - the forcible transportation outside of due process, characterised by Lord Steyn as 'kidnapping' in his Attlee Foundation lecture, and the invasive forms of interrogation - raise the most serious issues under international law.

Earlier this year, there were reports of a leaked memo from the Foreign Office to Number 10, revealing concern that Britain may have approved requests from the United States to permit extraordinary renditions. This is the background against which to assess Tony Blair's protestations of his commitment to fundamental rights and the rule of law, reflected in his email debate with Henry Porter in The Observer last month.

He attacked Porter's 'mishmash of misunderstanding, gross exaggeration and things that are just plain wrong', as he put it. Yet it is the PM who is prone to errors of fact: for example, under the Human Rights Act, for which his government deserves credit, judges are not, as he claimed, empowered to strike down acts of parliament. It is the PM, too, who is prone to gross exaggeration: the conditions of the modern world cannot be said to be such that 'traditional processes' are inadequate, as he claims.

And it is he who is prone to misunderstanding when he says that his approach reflects 'a genuine desire to protect our way of life from those who would destroy it'. Our 'way of life' includes our system of values, which includes a commitment to the rule of law. Returning foreigners to near-certain torture is not consistent with this. Aiding and abetting the transfer of British nationals and residents to Guantanamo, if that has occurred, would not be consistent with our 'way of life' or our values. 'Whose civil liberties?' he asks. Everyone's, we should respond.

It is difficult, therefore, to see the thread of principle that drives the Prime Minister in these written utterances.

It is equally difficult to see the principle behind his intemperate and inappropriate comments on the High Court judgment delivered last week on the right of the Afghan hijackers to stay in this country, when he claimed: 'It is not an abuse of justice for us to order their deportation. It is an abuse of common sense, frankly, to be in a position where we can't do this.'

If this is indicative of the PM's aim of protecting 'civil liberties for the majority', it is he who is out of touch with fundamental values and poses the more fundamental threat to constitutional democracy, the separation of powers and fundamental rights.

We do not yet know all the facts on extraordinary rendition or of Britain's possible involvement. Certainly, extraordinary rendition carried out in any form would be contrary to international law and any individual involved in the practice, even at the highest level, would be open to international criminal charge.

Complicity, which could include turning a blind eye, may be a crime under international law. These are early days in understanding the precise relationship between the Bush administration's policies on detainee transfers and interrogations, the legal advice and the allegations of abuse at Guantanamo, Abu Ghraib, Bagram and elsewhere.

But, from the materials I have seen on decision-making in relation to Iraq, on the attitude to the proposed removal of people to countries where they may face torture, on the failure to condemn Guantanamo for more than four years, on the desire to allow English courts to admit evidence that may have been obtained by torture, on the indefinite detention without charge of certain foreigners who cannot be deported, on the evasive answers in relation to rendition, it would not surprise me if materials were eventually to emerge which could show involvement in decisions concerning the international transfer of British nationals or residents.

In such circumstances, neither the Almighty nor instinct would be available by way of a defence.

· This is an extract from the Justice's International Rule of Law Lecture 2006 'Extraordinary Rendition: complicity and its consequences', to be given by Professor Philippe Sands QC tomorrow, 6.15pm at Middle Temple Hall, London. The lecture is free. To reserve a place, email lectures@justice.org.uk